The authors are indebted to Professor Guoli Ding for stimulating discussions. Xujin Chen is supported in part by the National Science Foundation of China under Grant 10771209 and the Chinese Academy of Sciences under Grant kjcx-yw-s7. Wenan Zang is supported in part by the Research Grants Council of Hong Kong, the Overseas and Hong Kong Macau Young Scholars Collaborative Research Fund of the National Science Foundation of China, and Seed Funding for Basic Research of the University of Hong Kong.